The present invention relates to a semiconductor laser manufacturing method and a semiconductor laser, and more particularly, to a semiconductor laser manufacturing method and a semiconductor laser with a low device resistance.
In recent years, semiconductor lasers are used in the field of optical pickup. In particular, to achieve high-density recording, semiconductor lasers having a short luminous wavelength and using a group-III nitride semiconductor are in widespread use (Japanese Unexamined Patent Application Publication Nos. 2003-78215 and 2007-250637).
As an example of semiconductor lasers using a group-III nitride semiconductor, a semiconductor laser 300 disclosed in Japanese Unexamined Patent Application Publication No. 2003-78215 will be described. The semiconductor laser 300 is an inner stripe type light-emitting device using an AlN layer as a current constriction layer. FIG. 4 is a sectional view showing the configuration of the semiconductor laser 300.
The semiconductor laser 300 has a configuration in which an Si-doped n-type GaN layer 302 (Si concentration: 4×1017 cm−3; thickness: 1 μm), an n-type cladding layer 303, an n-type guide layer 304, a 3-period multiple quantum well (MQW) layer 305, a cap layer 306, and a p-type GaN guide layer 307 are stacked on an n-type GaN substrate 301. The n-type cladding layer 303 is made of Si-doped n-type Al0.1Ga0.9N (Si concentration: 4×1017 cm−3; thickness 2 μm). The n-type guide layer 304 is made of Si-doped n-type GaN (Si concentration: 4×1017 cm−3; thickness: 0.1 μm). The 3-period multiple quantum well (MQW) layer 305 is composed of an In0.15Ga0.85N (thickness: 3 nm) well layer and an Si-doped In0.01Ga0.99N (Si concentration: 1×1018 cm−3; thickness: 4 nm) barrier layer. The cap layer 306 is made of Mg-doped p-type Al0.2Ga0.8N. The p-type GaN guide layer 307 is made of Mg-doped p-type GaN (Mg concentration: 2×1019 cm−3; thickness: 0.1 μm).
A current constriction layer 308, a p-type cladding layer 309, and a contact layer 310 are stacked on the p-type GaN guide layer 307. The p-type cladding layer 309 is made of Mg-doped p-type Al0.1Ga0.9N (Mg concentration: 1×1010 cm−3; thickness 0.5 μm). The contact layer 310 is made of Mg-doped p-type GaN (Mg concentration: 1×1020 cm−3; thickness 0.02 μm). A p-type electrode 311 and an n-type electrode 312 are formed on and below this stacked structure.
Next, a method for manufacturing the stacked structure of the semiconductor laser 300 will be described. First, the n-type GaN substrate 301 is put in a growth system, and the temperature of the substrate is then increased while supplying NH3. When the temperature reaches a growth temperature, a first crystal growth is started. In the first crystal growth, the Si-doped n-type GaN layer 302, the n-type cladding layer 303, the n-type guide layer 304, the 3-period multiple quantum well (MQW) layer 305, the cap layer 306, and the p-type GaN guide layer 307 are sequentially deposited. After that, the substrate temperature is decreased to a predetermined temperature, and a low temperature AlN layer is deposited. After the first crystal growth is finished, an opening 308a is formed in the low temperature AlN layer by photolithography and etching, to thereby form the current constriction layer 308.
After that, a second crystal growth is carried out. In the second crystal growth, the substrate temperature is increased to a growth temperature of 1100° C. with an NH3 feed rate of 0.36 mol/min, and the p-type cladding layer 309 is deposited. Then, the substrate temperature is decreased to 1080° C. and the contact layer 310 is deposited.
The low temperature AlN layer forming the current constriction layer 308 is formed in an amorphous state by the first crystal growth. Accordingly, the opening 308a can be easily formed in the low temperature AlN layer by wet etching to obtain the current constriction layer 308. The current constriction layer 308 is heated at a high temperature in the second crystal growth, and is crystallized by solid-phase growth by inheriting the crystal orientation of the GaN layer formed below the current constriction layer 308.
In the second crystal growth, the crystal growth progresses also from the surface of the crystallized current constriction layer 308, so that an even embedded shape can be obtained. When GaN or AlGaN having a low Al composition is formed on the crystallized current constriction layer 308, dislocations occur at a high density of 5×1010 to 1×1012 cm−2 due to a difference in lattice constant. The high-density dislocations alleviate a lattice strain in the current constriction layer 308, thereby obtaining an even surface with no cracks or pits.
In the inner stripe type semiconductor laser like the semiconductor laser 300, the contact width of the p-type electrode 311 can be set to be greater than the width of the opening 308a of the current constriction layer 308. Accordingly, also in the semiconductor laser provided with the opening 308a having a narrow width of 1 to 2 um for allowing a horizontal mode control, a low contact resistance is obtained, and thus a semiconductor laser with a low device resistance can be achieved.
Additionally, Japanese Unexamined Patent Application Publication No. 2007-250637 proposes a group-III nitride semiconductor light-emitting device that prevents deterioration in laser characteristics by preventing a cleavage failure from reaching the active layer.